This invention relates generally to semiconductor wafer polishing, and in particular to a method for joining sections of wafer polishing pads to form a larger, compound pad that is flat and inhibits leakage of polishing fluid.
Semiconductor wafers are generally prepared from a single crystal ingot, such as a silicon ingot, which is sliced into individual wafers. Each wafer is subjected to a number of processing operations to facilitate the installation of integrated circuit devices and to improve their yield, performance, and reliability. Typically, these operations reduce the thickness of the wafer, remove damage caused by the slicing operation, and create a flat and reflective surface. Chemical-mechanical polishing of semiconductor wafers is one technique to planarize wafer surfaces. It generally involves rubbing a wafer with a polishing pad in a solution containing an abrasive and chemicals, such as a colloidal silica and an alkaline etchant, to produce a surface that is extremely flat, highly reflective, and damage-free.
To maximize throughput in the preparation of semiconductor wafers, polishing machines are used to polish many wafers simultaneously. Polishing machines typically hold between 15 and 30 wafers in carriers that move relative to a rotating circular turntable, or platen, which is overlaid with a polishing pad. A stream of polishing solution, or slurry, is dispensed onto a surface of a pad while it is pressed against the wafers. Single-side polishing machines have one platen for polishing one side of wafers, while double-side polishing machines have two platens to simultaneously polish upper and lower sides of wafers. The platens are typically made of cast iron, and polishing pads are typically made of a polyurethane impregnated polyester felt having thickness between 1.5 and 2.0 mm. Pads are adhered to platen surfaces by an adhesive backing. The platen and polishing pad must be extremely flat to ensure that polished wafers are likewise extremely flat. During polishing, the wafer carriers and platen rotate in opposite directions for a predetermined time, a typical duration being 50 minutes.
Polishing machines that have platens of relatively large size are capable of polishing a greater quantity of wafers, thereby improving throughput relative to smaller platens. Machines with a platen diameter as great as 2000 mm are being used by silicon wafer manufacturers. However, pad manufacturers have generally not produced circular pads with diameter greater than about 1500 mm, nor produced rectangular pads that are sufficiently large to cut a 2000 mm diameter circle therefrom. As a result, smaller sectional polishing pads must be joined together to form a compound pad of larger size. Typically two semi-circular shaped pads are joined at a seam located along a diameter to form a circular-shaped compound pad.
Unfortunately, a seam where sectional pads are joined together is subject to leak. Polishing slurry can pass through gaps where a seam is not fully sealed to an underside of the pad where it contacts the platen. Moisture in the slurry causes the platen surface to quickly oxidize. Rust forms and contaminates the slurry, often spreading in the slurry back through the seam to the front side of the pad, where the rust diminishes pad life and causes iron contamination on wafers that substantially degrades wafer surface quality.
One potential solution is to seal seams with an applied bonding material, such as a spray sealant, to prevent slurry from passing through gaps and contacting platens. However, a sealant adds foreign material to a pad that is not uniformly distributed, creating local bumps and regions of irregular pad flexibility that degrade the pad's effective flatness, and subsequently degrade the flatness of wafers that are polished by the pads. Thus, sealing seams with applied bonding material is an inadequate solution.